It has been recently proposed that a particulate filter for capturing particulates in exhaust gas is incorporated in an exhaust pipe and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as reducing agent (additive agent) being sprayed at a position between the selective reduction catalyst and the particulate filter, thereby attaining lessening of both the particulates and NOx.
Such addition of the urea water to the selective reduction catalyst is conducted at the position between the particulate filter and the selective reduction catalyst. Thus, in order to ensure sufficient reaction time for pyrolysis of the urea water added to the exhaust gas into ammonia and carbon dioxide gas, it is necessary to prolong a distance between the urea-water added position and the selective reduction catalyst. However, arrangement of the particulate filter and the selective reduction catalyst in a substantially spaced apart relationship will extremely impair the mountability on a vehicle.
In order to overcome this, an exhaust emission control device compact in size as shown in FIG. 1 has been proposed. In the exhaust emission control device illustrated, incorporated in an exhaust pipe 2 through which exhaust gas 1 from an engine flows is a particulate filter 3 housed in a casing 5 to capture particles in the exhaust gas 1; arranged downstream of and in parallel with the particulate filter 3 and housed in a casing 6 are selective reduction catalysts 4 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. An exit end of the particulate filter 3 is connected to an entry end of the selective reduction catalysts 4 through an S-shaped communication passage 7 such that the exhaust gas 1 discharged from the exit end of the particulate filter 3 is reversely curved back into the entry end of the adjacent selective reduction catalysts 4.
The communication passage 7 is the S-shaped structure comprising a gas gathering chamber 7A which encircles the exit end of the particulate filter 3 to gather the exhaust gas 1 just discharged from the exit end of the particulate filter 3 through substantially perpendicular turnabout of the gas, a mixing pipe 7B which extracts the gathered exhaust gas 1 from the chamber 7A in a direction reverse to the flow of the exhaust in the particulate filter 3 and a gas dispersing chamber 7C which encircles the entry end of the selective reduction catalysts 4 so as to disperse the gas 1 guided by the mixing pipe 9B through substantially perpendicular turnabout of the gas into the entry end of the selective reduction catalysts 4. The gas gathering chamber 7A is provided with an injector 8 for addition of urea water into the exhaust gas 1 discharged from the exit end of the particulate filter 3 and flowing in the substantially perpendicular turnabout.
In the example illustrated, arranged in the casing 5 and in front of the particulate filter 3 is an oxidation catalyst 9 for oxidization treatment of the exhaust gas 1, and arranged in the casing 6 and behind the paired selective reduction catalysts 4 is an ammonia lessening catalyst 10 for oxidization treatment of surplus ammonia.
With such construction being employed, particulates in the exhaust gas 1 are captured by the particulate filter 3. The urea water is added downstream of the filter and intermediately of the mixing pipe 7B into the exhaust gas 1 by the injector 8 and is pyrolyzed into ammonia and carbon dioxide gas, so that NOx in the exhaust gas 1 is favorably reduced and depurated by the ammonia on the selective reduction catalysts 4. As a result, both the particulates and NOx in the exhaust gas 1 are lessened.
In this case, the exhaust gas 1 discharged from the exit end of the particulate filter 3 is reversely curved back by the communication passage 7 into the entry end of the adjacent selective reduction catalysts 4 so that a long distance is ensured between the urea-water added position intermediately of the communication passage 9 and the selective reduction catalysts 4 to ensure enough reaction time for production of ammonia from the urea water.
Moreover, the particulate filter 3 is arranged in parallel with the selective reduction catalysts 4 and the communication passage 7 is arranged between and along the particulate filter 3 and selective reduction catalysts 4 so that the whole structure becomes compact in size to substantially enhance its mountability on a vehicle.
As a prior art literature pertinent to this kind of exhaust emission control device compact in size, there already exists, for example, the following Patent Literature 1.